In 7DTD there are several server lists, Standard, Modded, Peer, Friends, History and Lan below is a short description of each tab

Standard - These are servers that have 0 mods and use the "reccomended" server settings
Modded - These are servers which EITHER differ from the "reccomended" server settings or have mods installed
Peer - These are servers which run off of people's pc's and can be laggy
Friends - These are servers where you have steam friends playing on
History - Servers you have played on in the past
Lan - Servers running on your home network

Its worth noting that if you want to appear on the "standard" list you have to use all the settings the devs specify, any deviation will put you on the modded list. You can see what the standard settings should be by loading up your server then looking at the output log within 7DaysToDieServer_Data in file manager

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There is a list of all the options in the settings, including two separate sections labeled "Standard Options" and "Modded Options" so you can clearly see which options are going to flag your server as "Modded"